1. Field of the Invention
This invention relates to the formation of a high temperature superconducting film on a substrate. More particularly, this invention relates to a process for the formation of a biaxially textured intermediate layer over a non-single crystal substrate to provide biaxial orientation which permits subsequent epitaxial growth of a biaxially oriented high temperature superconductor film thereover.
2. Description of the Related Art
Pulsed laser deposition is a demonstrated technique for fabricating high temperature superconducting thin films with excellent resistive transitions (T.sub.c) and high critical current densities. This technique offers several desirable characteristics for film fabrication, including rapid deposition rates, congruent material transfer, and simple target requirements. In addition, because photons induce material removal, a variety of films can be deposited without the need for a specialized atmosphere. Such pulsed laser deposition techniques are described, for example, by Russo et al. in "Metal Buffer Layers and Y--Ba--Cu--O Thin Films on Pt and Stainless-Steel Using Pulsed Laser Deposition", published in J. Appl. Phys. 68 (3), 1 Aug. 1990 at pages 1354-1356, and by Russo et al. in "Fabrication and Characterization of Y--Ba--Cu--O Thin films on Stainless Steel Substrates", in a paper submitted for publication in High Temperature Superconducting Compounds II, edited by S. H. Whang (Minerals, Metals, and Materials Society, Warrendale), 1990, pp. 1-6.
Most such high temperature superconducting thin films have been grown on single crystal substrates which promote the growth of oriented epitaxial films, and the resulting structures are suitable for electronic applications. However, such single crystal substrates are not suitable for conductor applications, such as electric power transmission; or energy storage using magnetic tapes.
Instead, metal substrates are usually used for such applications. However, to avoid metal migration from the substrate into the superconducting film (which can destroy the film's superconducting properties) an intermediate layer is usually formed over the metal substrate before depositing the superconducting film. This, in turn, has created problems, however, in attempting to achieve the same degree of axial orientation of the superconducting film as obtainable with single crystal substrates.
The formation and use of such intermediate layers is described, for example, in the aforementioned Russo et al. articles, which describe the use of an Ag intermediate layer. The use of a yttrium-stabilized zirconia (ZrO.sub.2) intermediate layer, commonly referred to as a YSZ layer, is described by Narumi et al. in "Critical Current Density in YBa.sub.2 Cu.sub.3 O.sub.6.8 Films on Buffered Metallic Substrates", published in Appl. Phys. Lett. 58 (11), 18 Mar. 1991, at pages 1202-1204; by Reade et al. in "Characterization of Y--Ba--Cu--O Thin Films and Yttria-Stabilized Intermediate Layers on Metal Alloys Grown by Pulsed Laser Deposition", published in Appl. Phys. Lett. 59 (6), 5 Aug. 1991, at pages 739-741; and by Kumar et al. in "Synthesis of Superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Films on Nickel-Based Superalloy Using In Situ Pulsed Laser Deposition", Appl. Phys. Lett. 57 (24) 10 Dec. 1990, at pages 2594-2596.
More recently a process has been described for forming a biaxially aligned YSZ intermediate layer by rf sputter deposition of the YSZ material on a substrate while bombarding the substrate with an ion beam of argon and oxygen. This permitted subsequent deposition of an in-plane aligned film of YBa.sub.2 Cu.sub.3 O.sub.7-X (YBCO) on the biaxially aligned YSZ layers by laser ablation. This process is described by Y. Iijima et al. in "In-Plane Aligned YBa.sub.2 Cu.sub.3 O.sub.7-X. Thin Films Deposited on Polycrystalline Metallic Substrates", published in Appl. Phys. Lett. 60 (6), 10 Feb. 1992, at pages 769-771.
While the authors of the above mentioned article on sputter deposition reported advantages from the combination of a rf sputter deposition with a simultaneous oxygen/argon ion bombardment, it has been found to be difficult to control an ion bombardment process and a sputter deposition process being simultaneously carried out in the same chamber, since the various parameters which must be controlled for proper sputter deposition, such as gas composition and flow rates, bias voltage, and pressure, are affected by the ion beam in the chamber. Therefore, such a modified sputter deposition process is not deemed to be a practical solution to the problem of forming a biaxially oriented intermediate layer over a non-single crystal substrate such as a metal substrate.
It would, therefore, be desirable to provide an easily controllable deposition process to deposit a biaxially oriented intermediate layer on a substrate without the use of a sputter deposition technique.